<style media="screen">
    #mask {
        display: none;
        position: fixed;
        top: 0;
        left: 0;
        height: 100vh;
        width: 100vw;
        background: #000;
        background: rgba(0,0,0,.6);
        z-index: 2;
    }
</style>

<div id="mask"></div>

<script>

</script>
